1. Field of the Invention
The present invention relates to a high-carbon steel wire rod to be made into steel wires for tire reinforcement, steel wires for prestressed concrete, and steel wires for ropes. The present invention relates also to a method for production of the same.
2. Description of Related Arts
High-strength steel wires are produced by drawing from high-carbon steel wire rods obtained by hot rolling. Those steel wire rods to be drawn into thin wires (such as tire cords and belt cords) need good drawability because their breakage at the time of drawing seriously impedes productivity. A conventional way to achieve good drawability was to subject hot wire rods to water quenching and ensuing air-blast quenching after hot rolling, thereby creating fine pearlite in the structure of the wire rods. Moreover, good drawability is ensured by intermediate patenting which is carried out once or twice during drawing.
There is a demand for high-carbon steel wires having a smaller diameter than before. Moreover, omission of intermediate patenting is required for improvement in productivity. Under these circumstances, high-carbon steel wire rods need good breakage resistance as well as good drawability for prolonged die life.
Japanese Patent Publication No. 60900/1991 discloses a technology to improve drawability by adequately controlling tensile strength per carbon equivalent in high-carbon wire rods and also by adequately controlling the ratio of coarse pearlite (distinguishable under a xc3x97500 microscope) in pearlite. Japanese Patent Laid-open No. 63987/2000 also discloses a technology to improve drawability by reducing the average diameter of pearlite colony below 150 xcexcm and by controlling the average lamella space between 0.1 and 0.4 xcexcm. The pearlite colony refers to a domain in which pearlite lamellas are oriented in one direction. A plurality of pearlite colonies form a nodule (or block) in which the crystal orientation is fixed. Incidentally, according to the above-mentioned patent publications, hot-rolled wire rods undergo water quenching for adequate winding temperature and subsequent air-blast quenching with a Stelmore conditioning cooling apparatus.
Unfortunately, the above-mentioned first technology does not provide sufficient breakage resistance as well as good drawability despite its contribution to prolong die life owing to the presence of coarse pearlite (about 10-30%) with a large lamella space. By contrast, the above-mentioned second technology contributes to prolonged die life on account of a larger lamella space (0.1 to 0.4 xcexcm); but such a large lamella space results in an average colony diameter of about 40 xcexcm (as illustrated in the example), which is detrimental to good drawability.
Incidentally, it has been reported that wire breakage is effectively prevented by increasing the lamella space and the pearlite nodule (block) size. (xe2x80x9cSeitetsu Kenkyuxe2x80x9d No. 295, pp. 520-63, 1978, issued by Nippon Steel Corporation) This report is based on the results of experiments with a high-carbon steel wire rod containing 1-2 wt % Cr. Moreover, it does not pay attention to the die life nor does it discuss the relation between the lamella space and the nodule size from the standpoint of drawability in relation to die life.
The present invention was completed in view of the foregoing. Accordingly, it is an object of the present invention to provide a high-carbon steel wire rod with superior drawability and a method for production thereof. The high-carbon steel wire rod has good resistance to breakage and contributes to prolonged die life.
The present inventors believed it essential for prolonged die life to enlarge the lamella space of pearlite to a certain extent, thereby slightly reducing the strength of wire rods. Based on this belief, they carried out extensive studies to suppress or prevent wire breakage. As the result, it was found that a wire rod has good breakage resistance and superior drawability so long as it contains pearlite nodules having an average diameter smaller than a certain value even though it has pearlite structure with a comparatively large lamella space. This finding led to the present invention.
The first aspect of the present invention resides in a high-carbon steel wire rod which has the chemical composition (in mass %) defined below:
C: 0.6-1.0%
Si: 0.1-1.5%
Mn: 0.3-0.9%
P: no more than 0.02%
S: no more than 0.03%
N: no more than 0.005%
with the remainder being Fe and inevitable impurities, and the structure which is characterized in that pearlite accounts for no less than 95 area % and pearlite has an average nodule diameter (P xcexcm) no larger than 30 xcexcm and an average lamella space (S nm) no smaller than 100 nm such that the value of F calculated by the formula below is larger than zero.
ti F=350.3/{overscore (S)}+130.3/{overscore (P)}xe2x88x9251.7
The chemical composition may additionally have either or both of the following components.
Nb: 0.020-0.050%
V: 0.05-0.20%
The chemical composition may have an optional component of Al in an amount no more than 0.030% and may contain N in an amount ranging from 0.0015 to 0.0050%.
The second aspect of the present invention resides in a method for producing a high-carbon steel wire rod which comprises the steps of subjecting a billet having the above-mentioned chemical composition to hot-rolling with a finish temperature of 1050-800xc2x0 C., cooling immediately the hot-rolled rod to a temperature of 950-750xc2x0 C. at a cooling rate no smaller than 50xc2x0 C./s, cooling further the rod to a temperature of 620-680xc2x0 C. at a cooling rate of 5-20xc2x0 C./s, cooling the rod for no less than 20 seconds at a cooling rate no larger than 2xc2x0 C./s. The above-mentioned method may have an additional step of further cooling the cooled rod to a temperature no higher than 300xc2x0 C. at a cooling rate no smaller than 5xc2x0 C./s.